With the practice in the orthopedic field, metal devices of titanium, stainless steel, and so forth are generally used as high-strength device for securing an injury (plates, screws, pins, etc.). Because these high-strength materials do not degrade in a living organism and remain in the living organism after fracture treatment, a second surgery is required to remove them. Thus, in order to eliminate the need for the second surgery, amorphous or semicrystalline bioabsorbent polymers, such as, for example, polyglycolic acid (PGA), polylactic acid (PLA), or the like, are used.
The ideal property of a biodegradable implant used in fracture treatment is that it should degrade and be replaced by bones within an appropriate amount of time. However, these materials do not have sufficient strength for use under high-load conditions, and there is a problem in that they cannot be used in applications in which high load is exerted thereon.
In response to the above-described problem, there have been known attempts to employ magnesium alloys as biodegradable implants (for example, see Patent Literature 1). Medical use of magnesium alloys has been considered because they have higher strength and are more biodegradable than PLA.
By undergoing chemical reactions with ions in water or body fluid in a living organism, magnesium alloys form calcium phosphate, which is a corrosion product, at the surface thereof, and also generate hydrogen gas. Once the surface of an implant is covered with the corrosion product, the rate of the above-described corrosion reaction drops, and the corrosion product, on the other hand, is phagocytosed by macrophages in the living organism. Due to this reaction in the living organism, the metal surface of the implant becomes exposed, and the biodegradation reaction on the implant is thought to advance due to the above-described corrosion reaction that advances over time.
Once the corrosion product is formed in a living organism, an implant made of a magnesium alloy takes a form whose influence on biological tissue is sufficiently low due to the suppressed generation of hydrogen gas; however, during an early stage in use of the implant before the corrosion product is sufficiently formed, a large amount of hydrogen gas is generated. In order to apply a magnesium alloy to an implant, this generation of hydrogen gas during the early stage in use of the implant should be suppressed, and as a means of achieving this, an anodic oxide layer is formed on the surface of a biodegradable implant to achieve passivation thereof (for example, see Patent Literature 2).